Following serum stimulation of quiescent 3T6 cells, an elevated in vivo rate of translation was observed. These studies were designed to identify the proteins associated with polysomal mRNA under different growth conditions in an attempt to establish a relationship between translational rate and the mRNA-associated proteins. Ultraviolet cross-linking of proteins to mRNA was employed to ensure that only genuine mRNA-associated proteins were investigated. Our results revealed little change in the population of mRNA-binding proteins, although minor variations in the synthesis of several proteins, most notably a 32 kilodalton species, were observed during growth transitions. These investigations demonstrate further that most of the mRNA-binding proteins were phosphorylated with the degree of phosphorylation of several proteins influenced by growth conditions.